# Unsteady Aerodynamic Characteristics depending on Reduced Frequency for a Pitching NACA0012 Airfoil at Rec=2.3×104

• Kim, Dong-Ha (Korean Air R&D Center) ;
• Chang, Jo-Won (Department of Aeronautical Science and Flight Operations, Korea Aerospace University) ;
• Sohn, Myong Hwan (Department of Aerospace and Mechanical Engineering, Cheongju University)
• 투고 : 2016.04.10
• 심사 : 2017.02.06
• 발행 : 2017.03.30

#### 초록

Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number $2.3{\times}10^4$. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

#### 참고문헌

1. McAlister, K. W. and Carr, L. W., "Water Tunnel Visualizations of Dynamic Stall", Journal of Fluids Engineering, Vol. 101, 1979, pp. 376-380. https://doi.org/10.1115/1.3448981
2. Ohmi, K., Coutanceau, M., Daube, O. and Loc, T. P., "Further Experiments on Vortex Formation Around an Oscillating and Translating Airfoil at Large Incidences", Journal of Fluid Mechanics, Vol. 255, 1991, pp. 607-630.
3. Carr, L. W., "Progress in Analysis and Prediction of Dynamic Stall", Journal of Aircraft, Vol. 25, 1988, pp. 6-17. https://doi.org/10.2514/3.45534
4. Chang, J. W. and Eun, H. B., "Reduced Frequency Effects on the Near-Wake of an Oscillating Elliptic Airfoil", Journal of Mechanical Science Technology, Vol. 17, 2003, pp. 1234-1245.
5. Ericsson, L. E., "Moving Wall Effect in Relation to Other Dynamic Stall Flow Mechanics", Journal of Aircraft, Vol. 31, 1994, pp. 1303-1309. https://doi.org/10.2514/3.46651
6. Kim, D. H. and Chang, J. W., "Unsteady Boundary Layer for a Pitching Airfoil at Low Reynolds Numbers", Journal of Mechanical Science Technology, Vol. 24, 2010, pp. 429-440. https://doi.org/10.1007/s12206-009-1105-x
7. Kim, D. H. and Chang, J. W., "Low-Reynolds-Number Effect on Aerodynamic Characteristics of a Pitching NACA0012 Airfoil", Aerospace Science Technology, Vol. 32, 2014, pp. 162-168. https://doi.org/10.1016/j.ast.2013.08.018
8. Kim, D. H. Chang, J. W. and Kim, H. B., "Aerodynamic Characteristics of a Pitching Airfoil through Pressure- Distortion Correction in Pneumatic Tubing", Journal of Aircraft, Vol. 50, 2013, pp. 590-598. https://doi.org/10.2514/1.C031941
9. Koochesfahani, M. M., "Vortical Patterns in the Wake of an Oscillating Airfoil", AIAA Journal, Vol. 27, 1989, pp. 1200-1205. https://doi.org/10.2514/3.10246
10. Bratt, J. B., "Flow Patterns in the Wake of an Oscillating Airfoil", Aeronautical Research Council, R&M 2773, 1953.
11. Katz, J. and Weihs, D., "Behavior of Vortex Wakes from Oscillating Airfoils", Journal of Aircraft, Vol. 15, 1978, pp. 861- 863. https://doi.org/10.2514/3.58463
12. Kim, J. S. and Park, S. O., "Smoke Wire Visualization of Unsteady Separation over an Oscillating Airfoil", AIAA Journal, Vol. 26, 1988, pp. 1408-1410. https://doi.org/10.2514/3.10056
13. Ho, S., Nassef, H., Pornsinsirirak, N., Tai, Y. C. and Ho, C. M., "Unsteady Aerodynamics and Flow Control for Flapping Wing Flyers", Progress in Aerospace Sciences, Vol. 39, 2003, pp. 635-681. https://doi.org/10.1016/j.paerosci.2003.04.001
14. Muller, T. J., "Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications", Progress in Astronautics and Aeronautics Series, 2001, pp. 586.
15. Kim, D. H., Chang, J. W. and Chung, J., "Low Reynolds Number Effect on the Aerodynamic Characteristic of a NACA 0012 Airfoil", Journal of Aircraft, Vol. 48, 2011, pp. 1212-1215. https://doi.org/10.2514/1.C031223
16. Laitone, E. V., "Wind Tunnel Tests of Wings at Reynolds Numbers Below 70000", Experiments in Fluids, Vol. 23, 1997, pp. 405-409. https://doi.org/10.1007/s003480050128
17. AIAA, "Assessment of Wind Tunnel Data Uncertainty", AIAA Standard S-071-1995, 1995.
18. Kim, D. H., Yang, J. H., Chang, J. W. and Chung, J., "Boundary Layer and Near-wake Measurements of NACA 0012 Airfoil at Low Reynolds Numbers", AIAA 2009-1472, 2009.
19. Brendel, M. and Muller, T. J., "Boundary-layer Measurements on an Airfoil at Low Reynolds Number", Journal of Aircraft, Vol. 25, 1987, pp. 612-617.
20. Theodorsen, R., "General Theory of Aerodynamic Instability and the Mechanism of Flutter", NACA report-496, 1935.
21. Garrick, I. E., "Propulsion of a Flapping and Oscillating Airfoil", NACA report-567, 1937.
22. Leishman, J. G., Principles of helicopter aerodynamics, Cambridge University Press, 2006.